Death and the Soul

Every living entity on the Material Plane, be it animal, plant, fungus, microorganism, or so on, contains a tether to the plane of Life, as is the Nature of the Goddes. The infrastructure of Magical Energy weaves through, allowing a multitude of these planes to interact as matter, energy, thought and feeling all combine. More complex creatures have more planar layers than others, so the density of the various planes is higher in their location. For example, a humanoid with memories and personality have more Soul (that is, their pocket of planar Life) than a blade of grass.
This effect is also present when comparing members of the same species to each other. A man who has lived a long and active life will be a better container for Life than a babe, but an active man in his prime will have more than an elderly man who has lived a quiet existence as a hermit. The exact proportions are complicated to work out, but it seems to be facets not just of physical health (as a dying body is a worse container), but also breadth and depth of personality (plane of Sentience) and maturity (plane of Emotion). Conventionally, the Life force is referred to as the “Soul” and the combination of emotional and intellectual factors that make up a person are the “Spirit”, though in most circumstances the distinction between these two isn’t notable.

Death and Dying

It is no surprise that complex biological creatures are mortal, eventually succumbing to predation, or running out of the ability to repair the damage to their own body that accumulates over time. While the process of death can occur for organisms as simple as a single cell, It is more complicated and interesting in the case of sentient beings. In brief, the physical process of dying has a beginning (for example, a lethal injury, or shutting down of one or more major organs due to illness or old age), where body tissue begins to die. Death is not an instantaneous process, as different areas of the body shut down at different times. One can easily see that a flower cut from its stem remains alive for a time after it has been disconnected from its life-giving body, and it will slowly wilt and deteriorate. The same is true for humanoids.
Once the body can be determined to be dead by such methods as lack of breathing, heartbeat, and brain activity, there are still living tissues inside that have yet to fully cease functioning. Also, the numerous colonies of microorganisms within and on the body do not all immediately die either. In addition, new organisms will quickly begin their own life cycles on the body, disconnected from the normal symbiosis — for example, flies lay their eggs. The line between life and death is a fuzzy one.
On the planar level, it is also gradual and indistinct. Absent of any intervention, the planes will begin to detach from their moorings in what was once a thinking individual. In a poetic sense, one might say that the life once lived as a distinct entity has in death returned to a complete and natural unity with all other living things. Emotion and Sentience are thought to retreat first, while Life is both first and last as it continually disengages from the original creature and enters those in the surroundings that are beginning to flourish. In a barren, frozen tundra,, or presumably the vacuum of space, Life energy will more slowly dissipate back into the plane without anything to tether to. The remaining planes (Mundane and Magical energies, Matter, and Change) decrease somewhat, but are noticeably quicker to dissipate into the surroundings and re-balance due to their pervasiveness not being dependent on Life.

Abnormal Death

Problems occur when the normal stages of death are interrupted or distorted. This has a small chance of occurring naturally, just like how cells copying DNA have a tiny chance of making a mistake which creates a mutation. It is a similarly tiny chance for a mistake to occur in the process of the planar layers detangling, which can result in the Soul staying tethered to the body, even though the body itself no longer has any life--it is merely a container for various microorganisms, and sometimes insects or fungi. This is apparently enough in some cases for the undead to be created naturally.
It is assumed that the risk of undeath, combined with inclinations of sentimentality, is the reason for the development of funeral customs among many sentient races. This is another humanoid ritual that, when properly performed, enforces the Effect of Belief, in this case the belief that the funeral will prevent undead from rising to terrorize their loved ones. War and natural disasters cause more undead due to the difficulty of retrieving the corpse and performing proper funerals for every death.
Sometimes, undead are created through necromancy. This is usually done in secret, and is one of the most universally frowned upon forms of magic, due to the instinctual aversion and great risk.

Tiers of Undeath

While technically any mortal entity, be it animal, plant, fungi, or so on, is capable of becoming undead, the simpler beings are less notable. They often expire quickly without the ability to gather more Soul, and without the layers of Sentience and Emotion present, it is more likely that Soul will spontaneously be able to detangle eventually. More complex beings, particularly sentient creatures, become the most interesting undead.
This is due to the Effect of Belief interplaying with the wide range of abilities given to the undead depending on the degree their Soul and Spirit are maintained. Once-sentient undead go through a period of waking, where the planar influences allow it to return to a limited period of consciousness. Other abilities may follow from there. Key to this process is the base level of Soul and Spirit available to them in life, and how much of that is remaining after awakening.
The exact mechanism is poorly understood, however there seems to be a ratio of Soul and Spirit that creates a Soular Capacity: the maximum amount of Soul and Spirit that the body can maintain a tether to, as if there is an invisible container available to be filled. While it is difficult to quantify (not least due to the hazards of research), observations of undead capable of using magic seem to operate at a fraction of the capacity of their once living personage.
The undead uses up their Soul and Spirit when performing actions that would be automatic and easy for the living, such as movement, thought, all the way to performing magic. This creates a sense of desparate hunger in the undead, as they are unable to recharge -- that is, draw more of the planes to them -- due to their death, as a living creature recharges through rest and the normal operation of their life.
The following tiers are arbitrary divisions arbitrary classifications designed to assist in handling an encounter with any undead, and allowing a better idea of how to combat their danger and spread. They proceed in order of progression from basic to advanced, with later tiers and stages of a tier occuring farther along in undead development and/or Soul. The lowest tier is exponentially more common than its higher tier. There is a fantastic variability from their particular personality, lifestyle, culture of origin, and circumstances of death.
It should be noted that the Soular Capacity generally indicates the tier an undead is capable of achieving, but their current amount of Soul determines the abilities they have at that particular moment. Some of these stages may overlap depending on the individual, for example a craftsman may find their skill in carpentry returned before their memories of their cousins, while a mother would more likely know their children before being capable of different emotions.

Animation

A living body is animated through the complex chemical and energetic reactions occurring in their cells. This is a process that anchors to and creates more Life. A dead body is normally no longer able to create or anchor the Life energy, however in undeath a corpse is able to remain an anchor, while no longer creating any Life.
The undead are able to animate themselves mainly as an echoing memory of life. Their physical form had an instinctual idea of how to operate. After Waking, their Spirit also retains first the limited memory of being a mobile creature, walking, swimming, or even flying just like its original form. It lacks finesse, however. An undead man can plod resolutely forward, but not dance or dodge an incoming blow. An undead hawk might fly in short bursts, but it is unable to glide on air currents.
This impression of poor mobility has drawn many humanoids to let their guard down. There is an advantage to their locomotion that creates a unique difficulty in halting their advancement. Their instincts are often fuzzy, and undead have been known to ignore hazards they once fled in their life. For example, it is common to observe undead walking straight through a fire, without a sense of fear, nor do they seem to exhibit pain from burning. Their flesh is damaged, but they don't realize it, allowing them to pursue their prey without stopping until their body is so injured it is incapable of any movement.
Disturbingly, the animation of the undead doesn't always correlate to the state of their body. This suggests that their locomotion is based on pure willpower, fueled by raw magic, rather than the mundane energy that powers the living. If a Tier I undead loses a leg in a bear trap, they can continue to move onward, appearing as if they have an invisible leg where their original has been detached. It is unknown if any undead are able to survive the complete destruction of their body, but it is theorized as an advanced Tier III ability.
More advanced undead who have regained some memories do avoid basic hazards, and can even be harmed by them once they are able to realize what the hazard means. For example, a Tier II undead avoids a fireplace, but if pushed in will recognize they are burning and attempt to escape. The key seems to reside in their perception of events and ability to retain this knowledge.

Tier I

1. uncontrollable hunger
2. able to speak, native language(s) first
3. personality traits, strongest to weakest
4. knows key relationships, oldest/strongest first
5. knows key life events/memories, strongest first
6. knows predominant daily routines (unskilled)

Uncontrollable hunger, commonly called Soul Hunger l is the first and most natural state of the undead, which is returned to once the current soul reaches a low threshold. It is not known if this is a set value, or proportional to the individual maximum capacity. All undead, regardless of their soul capacity, wake in the state of hunger due to the unfamiliarity of the sensation of what would more precisely be called soul-hunger, and the drain of the soul between life and undeath. Weaker undead never leave this state of raw hunger, and this is generally the case for most non-sentient animals.
Once soul hunger has been temporarily sated, it creates room for other traits to emerge. As always, these are general descriptions that will vary for each case. First, the undead will begin to create the impression that they are a sentient creature -- though in this Tier it is highly debated whether they are actually conscious. They will display common aspects of their personality from life, like being nervous when talking to strangers.
They will begin to remember their relationships with people they were close to, often beginning with their family. This often causes them to seek out their loved ones (to disastrous results). This often overlaps with recalling their memories, as noted in Memory in Soul and Spirit.
Finally, they might resume some of their common routines. For example, returning home and sweeping their floors, or attempting to farm, craft, and so on. However, their urges outstep their abilities if they are unable to move beyond this Tier, as their memories do not retain enough detail for them to actually know what they are doing. For example, they may try to cook dinner in the evening, placing their cauldron at the fire, placing objects that may or may not be real food inside, and poking around at the hearth where a fire should be. A common evidence of these undead are mundane objects being used in unconventional ways as if an infant tried to replicate what their parents do without instruction.

Tier II

1. able to use key skills to a lesser degree, best first
2. able to use all trained skills and key skills to full ability
3. able to use all skills untrained, like an average person
4. able to form memories
5. able to train new skills
6. able to use magic if known in life, or learned in undeath, with great ranges from least to most intricate

The ability to form memories comes late. Many undead may give the illusion of intelligence by calling for their loved ones, or navigating home. However, they often cannot hold real conversations or react fully to conditions. Uncautious proximity to those unconvinced of the danger may lead to uncontrolled feedings. Fortunately for the undead, most of them will not be able to remember doing so.
In this tier, the undead have some chance to abruptly recall non-key memories from their life. This ability is made easier by confronting stimuli related to that knowledge, as well as generally being more powerful. Once recalled, this knowledge is retained, but temporarily forgotten if they regress into Tier I.
For example, an undead man may instinctively return to his own home, and upon seeing his wife, he might remember her, perhaps along with some distinct happy memories of courtship or having their children. She, shocked by his appearance after death, might approach and call his name. In the effort of the journey, his reserves have been sapped by the effort of moving, and he reaches to consume her, overpowering her with his determined strength, a grip borne from magical force rather than pure physical ability. Afterward, his hunger sated, he recognizes her face again, and stays in the home with her corpse. He is unaware of the difference between her life and death, and has no urge for conversation or interaction.
For those undead that were skilled in magic during their life, their potential for use is available in the late states of this tier. However, many mages are unaware of the precise differences in power sources, and their ability lacks finesse after death. As a consequence, undead magic often uses the soul itself as a source of energy, which would not be recharged naturally without feeding. However, undead are able to gain soul directly--as opposed to the indirect way mortals recharge their soul by resting. The implications for living wizards have great potential for study, but in most areas this research is discouraged due to ethical concerns.

Tier III

1. detachment of cultural feeding, may use any method they know of
2. direct soul stealing
3. possibility to create other undead
4. able to survive destruction of the body

Unlike the previous tiers, each milestone of Tier III requires a great epiphany, and is not reached automatically, regardless of the soul capacity. They can be learned by anyone in theory, but--similar to advances in philosophy--it is an exceptional individual who reaches these states on their own without research or guidance. Even upon the realization, acting on these abilities may require elaborate rituals.
This tier is more difficult to summarize than the previous, not only due to its complexity, but also its rarity. Undead who reach this tier can sometimes pass for the living, amassing followers or causing an area, like a forest, to gain terrible reputations. They are shrouded in mystery, pushing the boundary of understanding for their unnatural kin.
The first hurdle they have to overcome is the realization of how their own nature now operates, which allows them to learn from other undead, or even the stories thereof, in order to expand their feeding ability beyond their own culture.
Next, they must understand the Effect of Belief and learn to harness it. Not just to think of it in theory, but to viscerally absorb this knowledge, the subtle anarchy of reality, into their being. Once this happens fundamentally, they become incredibly dangerous, able to siphon Life energy directly from their surroundings without fussing with the physical form of the victim, or perhaps even proximity.
The next stages are currently theoretical. It is thought that an advanced undead who is able to leech soul directly might learn to channel it into others, creating more undead who can do their bidding, or perhaps merely spreading their own soul into several containers.
If any undead being is able to survive the destruction of their physical form, then it is thought that this must be at only the highest tier. No one knows what could result from this aberration -- perhaps even a new godde!

Type

Metaphysical, Supernatural